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Songbird : Choosing The Output Tranny

Actually, it is already chosen. It is the existing 8k:8 center tapped primary that is already in the Crestline amp. However, I am excited by the choice, and at the risk of sounding like a dangerous lunatic, I’ll explain why.

First though, lets look at a standard SE output section.

Champ output section

That’s from a late 50’s (5F1) Champ, but they are all the same until the Medalist Skylark shows up. I’ve cleaned it up a little of the wires we don’t care about. So Vs is the highest voltage in the amp. It’s 305 to 315 in most of the old schematics and 375 for the 50’s Gibsons if the schematic is to be believed. Vs flows through the primary of the OT and into the plate of the output tube. Those are DC voltages (in theory) and in a perfect transformer, this would have zero loss and zero effect on the secondary.

So the DC voltage runs through the transformer to the plate. We are now past my understanding of the transformers action in one regard. Transformers with Vs through them are called energized and I’m not sure why this is done. With no signal, that is DC and there should be no energy in the magnet or coil. But amp guys always talk about an ‘energized’ transformer. I’ll dig into the RDH4 soon and maybe get an answer.

I would have thought that it would be better to capacitively couple the transformer, which I would have thought would result in lower requirements for the transformer.. but ok. What do I know?

As I said, in theory, this is a DC voltage but in fact it is only after the first filter capacitor. You might recall that there is significant ripple on the DC from the first filter (see here where I got 10 V pp on a 260VDC line). I wonder if that alone isn’t enough to create output hum.

So anyway.. signal comes in, gets amplified and shows up on the plate of the output tube. This varying current energizes the tranny which transfers energy to the speaker. The screen voltage is set to a value slightly lower than the plate for maximum output. And you must always have resistor (1k will do) on that screen.

However, the tube/tranny area is fertile ground for amp tweaking, primarily by lowering the screen voltage. There are a few ways to do this but I’ve bored you enough for now.